We want tilt Sensing
it's the Software
All these apps' watercolor effects are local in that the ink only moves in your strokes' vicinity. You will miss the global effects (and fun!) you get when using real watercolor which can flow pretty much all over the whole paper.
Now, take a look at the two videos of Expresii below. The first one was done on an 8" Atom-Z3735-based windows tablet (a 2014 model), which is too slow for a nice painting experience. The second one was on a 10.6" CORE-M based windows tablet released in the first half of 2015, which I'd already consider faster enough. (Update 2016: now with the latest version of CORE-M processor Expresii runs really fast enough on a mobile device)
In conclusion, brush/ink/watercolor sim is not easy. With good hardware, we still need good software to run on them to unleash the full potential.